1. Field of the Invention
This invention generally relates to plugs for pressure bodies; more specifically it relates to threaded expansion plugs.
2. Description of Related Art
It is often necessary to seal cavities or holes made through the wall of pressure vessels. For example, holes are often placed in hydraulic manifolds used in oil and gas wells. Sealing these holes can be accomplished through welding and use of plugs. There are many situations in which welding is undesirable, as it weakens the metal around the weld, or when welding is a safety risk due to the presence of flammable vapors—a common condition in the production of oil and natural gas.
Plugs are a common method of sealing cavities made through the surface of pressure vessels. The “NPT” style plug is commonly used. NPTs have a conical tapered thread to assist in sealing. While useful, the NPT is relatively large, and cannot be used in every application. NPTs are also prone to leakage when installed, as the threads provide the only seal.
Expansion plugs, such as the ball plug commercially available from Farmington Engineering of Madison, Conn., are another sealing solution. The Farmington Plug, viewable at www.farmingtoneng.com, discloses a cylindrical cup having a series of beveled shoulders, a conical closed end, and a ball expansion means secured in the open end of the cup. This plug is placed in an as-drilled hole, requiring no reaming. To seat the plug, the ball is struck with a special tool, forcing the ball further into the cup. The ball is compressed, and provides a radial force that assists in sealing.
U.S. Pat. No. 5,160,226, titled “Tapered Expansion Sealing Plug” discloses a tapered expansion sealing plug. One embodiment, a “pin plug,” is a frusto-conical plug body with peripheral U-shaped grooves and a frusto-conical pin expansion means. Another embodiment is a frusto-conical plug body with a ball-shaped expansion means.
Current expansion plugs, whether tapered or cylindrical, have one radial seal to prevent failure. Cylindrical expansion plugs consistently undergo catastrophic failure at high pressure. At 25,000 psi, for example, the plug not only leaks—it becomes a high-velocity projectile, creating an unsafe condition. There is a standing need for plugs that can withstand high pressure.